1. Technical Field
The present invention relates to a combustion engine in which the emission level of nitrogen oxides emitted to the environment is reduced by means of injecting urea into the exhaust pipe. The present invention also relates to a vehicle utilizing such an engine.
2. Background of the Invention
Diesel and petrol engines that are designed for lean operation, so-called lean-burn engines, exhibit good properties where fuel economy is concerned, though in normal operation generate particles and nitrogen oxides, NOx. A number of previously known methods can be utilized in order to reduce the amount of NOx in the exhaust gases. For instance, the combustion process can be cooled down by means of recirculation of exhaust gases, or the introduction of cooling medium in the form of water into the combustion chambers can be performed. These processes are utilized in order to reduce the formation of NOx in the combustion process. Another possibility is to reduce the amount of NOx in exhaust gases that have already been formed. One method for reducing the NOx content occurring in exhaust gases is to reduce NOx in a selective, reducing catalytic converter where reduction of NOx takes place influenced by urea. Accordingly, for this purpose, urea is injected into the exhaust pipe of the combustion engine, whereafter reaction takes place in a reaction chamber.
An example of a device for NOx-reduction at a combustion engine is disclosed in U.S. Pat. No. 5,976,475, the disclosure of which is hereby incorporated herein by reference. In order to obtain high efficiency of the injected urea quantity in relation to the amount of NOx present in the exhaust pipe, it is required that the injected urea be mixed thoroughly with the exhaust gases. This is of particular importance, since the time of exposure inside the exhaust pipe is short as a result of the relatively high flow rate through the system. In the device disclosed in the above-mentioned document, urea is injected upstream of a turbine located in the exhaust pipe in order to achieve a thorough mixing of exhaust gases and urea.
As a result of the positioning of the injection site upstream of the turbine, the injected urea passes through the entire turbine housing and, accordingly, also in the vicinity of the rotational shaft of the turbine wheel that is mounted in bearings in the housing enclosing the turbine wheel. The rotational shaft is mounted in bearings in a through-hole in the housing. When the urea passes this through-hole, there is a risk of urea penetrating the bearing assembly of the rotational shaft in the housing, since the pressure inside the housing is higher than the ambient pressure. Since urea is very reactive and aggressive, there is a risk of the urea degrading the bearing assembly, seals and lubricant that are present therein.